A flyback switching power converter is typically provided with a mobile device as its transformer provides safe isolation from AC current. Like all switching power converters, a flyback includes a power switch transistor that is controlled by a controller to regulate the power delivery to the load. The cycling of the power switch creates switching noise that can affect the mobile device. A mobile device often includes a touchscreen that is becoming more and more sensitive to switching noise. For example, users can now operate touchscreens on many phones and tablets even when wearing gloves. The downside to such high sensitivity touchscreens, and other electrical components, is that the operation of these components can be susceptible to interference from the switching noise. For example, sensing where a touchscreen has been touched involves a touchscreen sensor monitoring certain frequency bands as described in detail below. If the electromagnetic interference (EMI) produced by a switching power converter is within the frequency band monitored by the touchscreen sensor, then performance of the touchscreen may be undesirably altered. It will be appreciated that the performance of other electrical components may also be undesirably altered by the EMI produced by switching power converters.
The interference from switching noise from a switching power converter such as a flyback is exacerbated because of the various switching modes used to increase efficiency. In particular, it is conventional to cycle the power switch using pulse width modulation (PWM) during periods of relatively heavy load. The duty cycle (pulse width) is reduced as the load is reduced during PWM operation. But as the load continues to reduce, it is more efficient to cycle the power switch using pulse frequency modulation (PFM). The various switching frequencies used during PFM operation (e.g., from 22 KHz to 89 KHz) spreads the switching noise across a relatively wide frequency band such that finding a suitably noise-free band for touchscreen operation may be problematic.
Accordingly, there exists a need to control the frequency bands of EMI produced by switching power converters.